1. Field of the Invention
The present invention relates to a press-cushion sheet. More particularly, the present invention relates to a press-cushion sheet usable for hot-press machines for producing laminate plates, for example, decorative laminates or printed circuit boards.
2. Description of the Related Art
In a press-molding process by a hot press machine, a press-cushion sheet is arranged between a heating plate and a molding plate, to uniformly apply a heat and pressure to the surface of the material to be press-molded and to correct a stress of the pressed material derived from the heating plate.
In the conventional process, various felt sheets, rubber sheets and kraft paper sheets are employed as press-cushion sheets, but it is known that the felt press-cushion sheet does not have a uniform density distribution, and thus it is difficult to apply a uniform press-heating treatment to the material to be pressed. Also, the rubber press-cushion sheets are easily deteriorated under high temperature pressing condition, for example, at a temperature of from 160.degree. C. to 220.degree. C. under a pressure of from 40 to 120 kg/cm.sup.2, and thus the cushioning performance of the rubber sheet is lowered and the rubber sheet per se is deformed. Further, since the kraft paper press-cushion sheet is provided by superimposing 10 to 20 individual paper sheets one on the other, and the individual paper sheets are frequently broken, a superimposing of the individual paper sheets and exchange of the broken paper sheets for fresh sheets become necessary, and thus the pressing procedure exhibits a poor operating efficiency.
Known press-cushion sheets other than the kraft paper press-cushion sheets are made from woven fabrics, nonwoven fabrics, felt sheets and synthetic paper sheets composed of organic fibers or inorganic fibers.
Japanese Unexamined Patent Publication No. 59-192,795 discloses a press-cushion sheet produced by laminating a plurality of wet synthetic paper sheets each composed of inorganic fibers having a length of 70 to 1000 .mu.m and aramid pulp particles having a freeness of 150 seconds or more but less than 500 seconds, and heat-pressing the resultant wet laminate.
Japanese Unexamined Patent Publication No. 58-7,646 discloses a press-cushion sheet comprising a hard cushion layer composed of a heat resistant fiber nonwoven fabric impregnated with a cured resin or rubber and a soft cushion layer composed of a heat resistant fiber nonwoven fabric bonded to a surface of the hard cushion layer through an adhesive layer, and having a releasing layer formed on the surface thereof.
Japanese Unexamined Patent Publication No. 55-101,224 discloses a heat-treated press-cushion sheet comprising a plurality of woven fabrics and plurality of pad fiber layers and laminated together by needle-punching.
The above-mentioned conventional press-cushion sheets are disadvantageous in that the production process is complicated and the durability of the resultant cushion sheet in practical use is not satisfactory.